Carrier aggregation or CA is one of the new features recently developed by the members of the 3rd-Generation Partnership Project, 3GPP, for so-called Long Term Evolution, LTE, systems, and is standardized as part of LTE Release 10, referred to as “LTE Rel-10” or simply “Rel-10”, which is also known as LTE-Advanced. Rel-8 is an earlier version of the LTE standards and it supports bandwidths up to 20 MHz. In contrast, LTE-Advanced supports bandwidths up to 100 MHz. The very high data rates contemplated for LTE-Advanced require an expansion of the transmission bandwidth.
However, to maintain backward compatibility with Rel-8 mobile terminals, the available spectrum in Rel-10 is divided into chunks called component carriers, or CCs, where each CC is Rel-8 compatible. CA enables bandwidth expansion beyond the limits of LTE Rel-8 systems by allowing mobile terminals to transmit data over an “aggregation” of multiple Rel-8 compatible CCs, which together can cover up to 100 MHz of spectrum. This approach to CA ensures compatibility with legacy, Rel-8 mobile terminals, while also ensuring efficient use of the wider carrier bandwidths supported in Rel-10 and beyond by making it possible for the legacy mobile terminals to be scheduled in all parts of the wideband LTE-Advanced carrier.
The number of aggregated CCs, as well as the bandwidth of the individual CCs, may be different for uplink, UL and downlink, DL, transmissions. The configuration of aggregated CCs is referred to as “symmetric” when the number of CCs in the UL is the same as in the DL. Thus, a CA configuration with different numbers of CCs aggregated in the UL versus the DL is referred to as an asymmetric configuration. Note, too, that the number of CCs configured for a geographic cell area may be different from the number of CCs seen by a given mobile terminal. A mobile terminal, for example, may support more downlink CCs than uplink CCs, even though the same number of uplink and downlink CCs may be offered by the network in a particular area.
LTE systems can operate in either Frequency-Division Duplex, FDD, mode or in TDD mode. In FDD mode, downlink and uplink transmissions take place in different, sufficiently separated, frequency bands. In TDD mode, on the other hand, downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum. TDD mode also allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively. In this regard, the UL/DL configuration of a TDD cell determines, among other things, the particular allocation of subframes for DL use and for UL use, within a given radio frame. Different UL/DL configurations correspond to different proportions of DL and UL allocations. Accordingly, UL and DL resources can be allocated asymmetrically for a given TDD carrier.
One consideration for operation in the CA context is how to transmit control signaling on the UL from a mobile terminal to the wireless network. Among other things, UL control signaling includes HARQ feedback. As used herein, the term “HARQ feedback” denotes the HARQ-ACK bits transmitted from the mobile terminal for the involved CCs, for a given HARQ feedback window. Here, the term “HARQ feedback window” refers to the overall set or span of DL subframes that is associated with the HARQ feedback being generated, as taken across all serving cells involved in the HARQ feedback generation. Moreover, the term “HARQ-ACK bit” is used to refer to a given HARQ feedback bit, regardless of whether the state of that bit is an ACK value, a NACK value, or a Discontinuous Transmission, DTX, value.
A User Equipment, UE, or other mobile terminal operating in accordance with LTE Rel-8 or Rel-9—i.e., without CA—is configured with only a single downlink CC and uplink CC. The time-frequency resource location of the first Control Channel Element, CCE, used to transmit the Physical Downlink Control Channel, PDCCH, for a particular downlink assignment determines the dynamic resource to be used by the targeted UE for sending corresponding HARQ feedback on a PUCCH, which in this context is referred to as a “Rel-8 PUCCH”. No PUCCH collisions occur in the Rel-8 scheme, because all PDCCHs for a given subframe are transmitted by the network using a different first CCE. Therefore, each targeted UE sends HARQ feedback corresponding to its PDCCH reception using different CCE resources in the UL.
HARQ feedback becomes more complicated in the CA context, where the HARQ feedback relates to multiple serving cells or, equivalently, multiple CCs. However, Rel-10 provides a number of defined approaches to sending such feedback. These defined approaches build in some sense on the approach used in Rel-8, but with certain multiplexing and timing provisions to cover the multiple cells/CCs involved in the HARQ feedback. Rel-10 procedures, however, assume that all serving cells in a given CA configuration have the same UL/DL configurations and thus have the same UL/DL subframe allocations.
Rel-11, among other things, adds the flexibility of aggregating carriers having different UL/DL configurations and aggregating carriers having different frequency bands and/or Radio Access Technologies, RATs. Rel-11 thus introduces a number of new HARQ feedback scenarios that are incompatible with the HARQ feedback signaling introduced in Rel-10 for CA scenarios.